1. Field of the Invention
The present invention generally relates to information storage apparatuses, and, more particularly, to an information storage apparatus that generates a tracking error signal based on a detection signal, and performs various control operations based on the generated tracking error signal.
2. Description of the Related Art
In an information storage apparatus such as an optical disk apparatus, a tracking error signal is generated based on a detection signal of reflected light. Based on this tracking error signal, tracking control and seek control are performed. In recent years, detection signals have become smaller with higher recording densities. As a result, the stability of the tracking control and seek control may deteriorate.
FIG. 1 is a block diagram of a conventional optical disk apparatus, mainly illustrating the tracking servo system of the optical disk apparatus.
The laser light emitted from a laser diode 110 is supplied to a collimator lens 111. The collimator lens 111 converts the laser light supplied from the laser diode 110 into a collimated beam. The collimated beam passes through a beam splitter 112, and is then supplied to a mirror 113. The mirror 13 reflects the incident light towards an objective lens 114. The objective lens 114 converges the beam from the mirror 113 onto a disk medium 115. The light beam converged onto the disk medium 115 is reflected from the disk medium 115, and then enters the beam splitter 112 via the objective lens 114 and the mirror 113.
The beam splitter 112 reflects the reflected light from the disk medium 115 towards an optical unit 116. The optical unit 116 comprises a Wollaston prism, a Foucault unit, and a detection lens. The optical unit 116 extracts various signal components. Among the extracted signal components, a tracking error signal component is supplied to a photodetector 117 that converts incident light into an electrical signal.
The electrical signal supplied from the photodetector 117 is supplied to a tracking error signal producing circuit 118. A tracking error signal produced by the tracking error signal producing circuit 118 is supplied to low-pass filters 119 and 120. The low-pass filters 119 and 120 remove noise. The tracking error signal from the low-pass filter 119 is supplied to a DSP (Digital Signal Processor) 123. The DSP 123 performs a phase compensation on the tracking error signal outputted from the low-pass filter 119 at the time of a tracking operation.
The tracking error signal outputted from the low-pass filter 120 is supplied to a comparator 121. The comparator 121 digitizes the tracking error signal. The digitized signal from the comparator is supplied to a counter 122 that counts the digitized signal. The counted value outputted from the counter 122 is supplied to the DSP 123.
Based on the counted value supplied from the counter 122 at a time of a seek operation, the DSP 123 detects the number of tracks the beam has traversed, and controls the moving speed of the light beam during the seek operation.
Also, based on the tracking error signal outputted from the low-pass filter 119, the DSP 123 controls a driving circuit 124. The driving circuit 124 drives an actuator 125. The actuator 125 moves the objective lens 114 in the direction of the arrow A, which is the radial direction of the disk medium 115. As the actuator 125 is moved in the direction of the arrow A, the light beam emitted onto the disk medium 115 wobbles in the direction of the arrow A, thereby performing tracking control.
FIG. 2 shows the circuit structure of the conventional tracking error signal producing circuit 118.
The tracking error signal producing circuit 118 comprises NPN transistors Q1 to Q6, resistors R1 and R2, voltage sources 131 and 132, a current source 133, and a differential amplifier circuit 134, which constitute a current computing-operated AGC circuit. This current calculating AGC circuit is disclosed in Japanese Laid-Open Patent Application No. 6-36303, for instance.
In the tracking error signal producing circuit 118 having the structure shown in FIG. 2, a tracking error signal TES is given byTES=R×Iref×(Ia−Ib)/(Ia+Ib)  (1)where R indicates the resistance value of the resistors R1 and R2, Iref indicates the current that passes through the current source 133, Ia and Ib indicate the currents supplied from the photo detector 117.
In the tracking error signal producing circuit 118 shown in FIG. 2, the tracking error signal TES is the ratio of a difference signal and a sum signal, which is not influenced by the light emission level during a recording operation or a reproducing operation.
When the input currents Ia and Ib become small, the input impedance becomes large. As the input impedance becomes large, the cutoff frequency of the low-pass filter, which is made up of the input impedance and the input capacitance, becomes lower.
The tracking error signal TES is used for track counting during the seek operation, as described above. Therefore, it is essential to satisfy the track zero-cross frequency at the maximum light moving speed when crossing the tracks. However, if the cutoff frequency decreases, the track zero-cross frequency might not be satisfied.
Also, in the pre-pit region in which ID signals and the likes are recorded, the amount of light reflected and returned from the disk medium 115 decreases. As a result, the tracking error signal becomes small, and the input currents Ia and Ib become even smaller.